An amplifier generally amplifies an input signal using an operational amplifier. An amplifier is unable to generate a voltage at a level equal to or higher than a supply-power voltage thereof. Therefore, voltage amplitude of an input voltage may be limited by installing a clamp circuit inside the amplifier. The clamp circuit limits the voltage amplitude based on a reference voltage INREF. Therefore, when a voltage level of the reference voltage INREF fluctuates, a voltage output from the clamp circuit (hereinafter called a “clamp voltage”) may not become symmetric in higher and lower sides than the reference voltage INREF.
In a case where the clamp voltage is not symmetric with respect to the reference voltage INREF, a difference is caused in charge and discharge current of a DC cut capacitance between a case where the input voltage is higher than the reference voltage INREF and a case where the input voltage is lower than the reference voltage INREF. As a result, a DC offset of an operational amplifier is caused.